无氧条件下Pd/Ce O_2催化剂表面NH_3还原NO过程中N_2O形成机理研究（英文）
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摘要
氮氧化物(NO_x,主要包括NO和NO_2)是主要的大气污染物之一,造成酸雨,光化学烟雾和臭氧层破坏等环境问题,甚至直接危害人体健康.化石燃料燃烧和汽车尾气排放是NO_x的主要来源,严格控制火力发电厂,大型锅炉,汽车尾气等污染源中NO_x的排放刻不容缓.以NH_3为还原剂选择性催化还原NO_x(NH_3-SCR)是目前公认的最有效的NO_x脱除技术,然而在催化NO_x还原为N_2的过程中往往伴随着副产物N_2O的生成,降低了催化剂的选择性,造成温室气体效应和破坏臭氧层等环境问题.因此充分理解NH_3-SCR过程中N_2O的形成机理对于抑制N_2O的产生、提高催化剂的选择性十分重要.本文将高度分散的Pd纳米团簇负载在Ce O_2纳米棒上制成Pd/Ce O_2催化剂,结合NH_3-TPD, NO-TPD和原位傅里叶转换红外光谱等表征手段研究了无氧条件下该催化剂上利用NH_3催化还原NO过程中N_2O的产生路径.结果表明, N_2O的形成途径与反应温度和反应气体的浓度相关.当反应气体中NH_3含量大于化学计量比时,在反应温度低于200°C时,由NH_3活化产生的吸附态H·自由基与催化剂表面吸附的NO反应先生成中间产物HON,两个HON分子进一步反应生成N_2O;过量的吸附态的H·自由基也可以与HON反应生成N_2,所以低温下(<200°C)随着反应气氛中NH_3的增加,解离生成的H·也随之增加,促进反应向着生成N_2的方向进行,从而抑制了N_2O的产生.随着反应温度增加, NH_3解离产生的H·被CeO_2表面的O捕获形成羟基,中间产物HON的生成被切断,从而阻断了N_2O的生成.同时由于体系中含有大量的NH_3,吸附态的NO会优先与活化态的NH_3物种反应生成N_2,阻碍了NO解离生成N_2O这一过程的发生,因此NH_3过量情况下在高温下观察不到N_2O的产生,可获得100%的N_2选择性.但是当反应气体中的NH_3含量不足时,即体系中含有过量的NO,当反应温度高于250°C, NO可在催化剂表面解离生成吸附态的N·自由基和O·自由基, N·自由基可进一步与吸附态的NO反应生成N_2O, NO的解离是N_2O生成的速控步,还原性吸附物种对O·自由基的捕捉将有利于N_2O的生成.当反应温度介于200–250°C, NH_3解离产生的H·自由基既可以与NO结合生成HON中间产物,又能被CeO_2表面的O捕获形成羟基,两个反应之间存在竞争,此时N_2O产生与反应气体浓度之间的关系不再呈单调变化.
N_2 O is a major by-product emitted during low-temperature selective catalytic reduction of NO with NH_3(NH_3-SCR), which causes a series of serious environmental problems. A full understanding of the N_2 O formation mechanism is essential to suppress the N_2 O emission during the low-temperature NH_3-SCR, and requires an intensive study of this heterogeneous catalysis process. In this study, we investigated the reaction between NH_3 and NO over a Pd/CeO_2 catalyst in the absence of O_2, using X-ray photoelectron spectroscopy, NH_3-temperature-programmed desorption, NO-temperature-programmed desorption, and in-situ Fourier-transform infrared spectroscopy. Our results indicate that the N_2 O formation mechanism is reaction-temperature-dependent. At temperatures below 250 °C, the dissociation of HON, which is produced from the reaction between surface H· adatoms and adsorbed NO, is the key process for N_2 O formation. At temperatures above 250 °C,the reaction between NO and surface N·, which is produced by NO dissociation, is the only route for N_2 O formation, and the dissociation of NO is the rate-determining step. Under optimal reaction conditions, a high performance with nearly 100% NO conversion and 100% N_2 selectivity could be achieved. These results provide important information to clarify the mechanism of N_2 O formation and possible suppression of N_2 O emission during low-temperature NH_3-SCR.
N_2 O is a major by-product emitted during low-temperature selective catalytic reduction of NO with NH_3(NH_3-SCR), which causes a series of serious environmental problems. A full understanding of the N_2 O formation mechanism is essential to suppress the N_2 O emission during the low-temperature NH_3-SCR, and requires an intensive study of this heterogeneous catalysis process. In this study, we investigated the reaction between NH_3 and NO over a Pd/CeO_2 catalyst in the absence of O_2, using X-ray photoelectron spectroscopy, NH_3-temperature-programmed desorption, NO-temperature-programmed desorption, and in-situ Fourier-transform infrared spectroscopy. Our results indicate that the N_2 O formation mechanism is reaction-temperature-dependent. At temperatures below 250 °C, the dissociation of HON, which is produced from the reaction between surface H· adatoms and adsorbed NO, is the key process for N_2 O formation. At temperatures above 250 °C,the reaction between NO and surface N·, which is produced by NO dissociation, is the only route for N_2 O formation, and the dissociation of NO is the rate-determining step. Under optimal reaction conditions, a high performance with nearly 100% NO conversion and 100% N_2 selectivity could be achieved. These results provide important information to clarify the mechanism of N_2 O formation and possible suppression of N_2 O emission during low-temperature NH_3-SCR.
引文
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[38]Q.Y.Liu,Z.Y.Liu,C.Y.Li,Chin.J.Catal.,2006,27,636-646.